The present invention relates to a method for the purification of solutions, in particular aqueous solutions, in order to remove pollutant ions, such as heavy metals and radio-nuclides therefrom.
The purification of water to remove heavy metals and radionuclides is one of the primary tasks required for environmental clean-up. In many cases the water contains quantities of other solid or liquid materials which it would be uneconomic to recover together with the contaminants, and it is therefore desirable to be able to remove selectively the toxic materials in question. Most commonly in water treatment an undesirable constituent is removed by absorbing it onto, or converting it into, a solid phase. If this is done the material can be removed by physical settling or "column" operation, if the particles are large, or filtration, if the particles are small.
In order to treat large flow rates of water in small sized plants it is necessary for the transfer of pollutant to a solid phase to take place rapidly, which infers that small particles will be desirable. Also, if the particles are non-porous they will need to be small to achieve an adequate surface to volume ratio, thereby achieving a reasonable capacity for the pollutant in question. However, the filtration of small particles is normally difficult and energy intensive.
Selective ion exchange is well established as a technique for removing selected pollutants from water, in particular chelating ion exchange, in which the metals are held by organic chelating groups attached to a solid organic polymer. The binding reaction is typically reversed by exposure to acid solutions.
It has been previously proposed to remove solid or liquid phases from liquid media by processes which involve a magnetic treatment.
For example, GB-A-2170736 describes the functionalisation of magnetite with sulphide groupings that attract heavy metals.
GB-A-2206206 describes a method of binding small particles reversibly to magnetic particles using a polyionic polymer to effect the binding, for the purposes of removing the small particles from solution. This method is particularly applicable to the clarification of solutions.
EP-A-0302293 describes the purification of solids and liquids by means of a granulate of magnetic particles mixed with a substance which absorbs the impurities to be removed. The granulate is produced by mixing the magnetic particles with the absorber and pressing the mixture.
U.S. Pat. No. 4,935,147 describes the separation of a substance from a liquid medium in which magnetic particles are coupled to non-magnetic particles by chemical means for non-specifically binding the particles together. The chemical means for binding the particles together may be, for example, a polyelectrolyte. The binding is reversible.
U.S. Pat. No. 4,134,831 describes a process for removing pollutants from lakes, rivers or ocean sediments in which a selective ion exchanges is mechanically attached to magnetic particles, for example by mixing the ion exchanger with the magnetic material and forming granules therefrom.
U.S. Pat. No. 4,661,327 describes a process for the removal of contaminants from soil by mixing the soil with a cation or anion resin polymerised on a magnetic core, followed by magnetic separation of the magnetic particles.